The Trial to Reduce Insulin Dependent Diabetes in the Genetically at Risk ('TRIGR') will determine whether weaning to a formula in which (foreign) proteins have been extensively hydrolysed, reduces disease risk for Type 1 Diabetes (T1D) in genetically susceptible children, as it does in rodent models. The Specific Aims are: I-a: To determine whether weaning to a hydrolysed casein formula (Nutramigen(tm)) reduces the frequency of diabetes-predictive autoantibodies, and I-b: To determine whether weaning to casein hydrolysate reduces the frequency of clinical diabetes. This double blind, randomized controlled trial in subjects with an affected first degree relative and risk-associated HLA genotypes, requires 2032 eligible infants: 4516 newborn babies need to be recruited, 45% of which will have eligible HLA genotypes (45.2% to date). An international, multicenter consortium has been developed comprising 73 centers in 15 countries. By the end of January'05 we achieved 65% of the recruitment target with 3187 infants registered, 2775 randomized and 1186 eligible infants entered into the intervention . The 6-8 month intervention is designed to compare the effects of either hydrolysed casein or standard cow milk based weaning formula. Duration of breast feeding is at the mothers'discretion. Recruitment and intervention will be completed by the second year of this proposal for trial continuation. All subjects are followed during and after the intervention period for 10 years with measurements of serological markers of intact cow milk exposure, diabetes predictive autoantibodies (the end point at age 6 years) and the clinical and/or metabolic indices of diabetes (the end point at age 10 years). A large, cross-linked repository of stored sera, DNA and cryopreserved peripheral blood mononuclear cells allows independently funded ancillary and mechanistic studies related to the natural history of prediabetes and the hypothesis to be tested. Cow milk protein is the most common intact foreign weaning protein in humans. If our intervention is effective in delaying autoimmunity or its progression to diabetes, this first ever primary prevention study of T1D will have far-reaching impact for individuals and the global society.